Ventilator-associated pneumonia: monotherapy is optimal if chosen wisely

Antibiotic Resistance Mechanisms in the Intensive Care Unit

Antibiotic resistance is increasingly recognised as a major threat to global health, with few new antimicrobial agents in development. The intensive care unit provides a unique environment for the growth and spread of drug-resistant organisms. Knowledge of the pathogenesis and mechanisms of resistance of drug-resistant organisms provides a conceptual framework which underpins the clinical manifestation of infections caused by these organisms, and is crucial for the intensivist to understand. Particular importance lies in the prevention of infection and the control of drug-resistant pathogens. The major resistance mechanisms of these organisms will be highlighted, focusing on specific gram-positive (meticillin-resistant Staphylococcus aureus and glycopeptide-resistant Enterococci), gram-negative ( Pseudomonas aeruginosa, Acinetobacter baumannii and multi-drug resistant Enterobacteriaceae) organisms, and then placed in historical and clinical context.

Pseudomonas aeruginosa: resistance and therapeutic options at the turn of the new millennium

Pseudomonas aeruginosa is a major cause of nosocomial infections. This organism shows a remarkable capacity to resist antibiotics, either intrinsically (because of constitutive expression of beta-lactamases and efflux pumps, combined with low permeability of the outer-membrane) or following acquisition of resistance genes (e.g., genes for beta-lactamases, or enzymes inactivating aminoglycosides or modifying their target), over-expression of efflux pumps, decreased expression of porins, or mutations in quinolone targets. Worryingly, these mechanisms are often present simultaneously, thereby conferring multiresistant phenotypes. Susceptibility testing is therefore crucial in clinical practice. Empirical treatment usually involves combination therapy, selected on the basis of known local epidemiology (usually a beta-lactam plus an aminoglycoside or a fluoroquinolone). However, therapy should be simplified as soon as possible, based on susceptibility data and the patient's clinical evolution. Alternative drugs (e.g., colistin) have proven useful against multiresistant strains, but innovative therapeutic options for the future remain scarce, while attempts to develop vaccines have been unsuccessful to date. Among broad-spectrum antibiotics in development, ceftobiprole, sitafloxacin and doripenem show interesting in-vitro activity, although the first two molecules have been evaluated in clinics only against Gram-positive organisms. Doripenem has received a fast track designation from the US Food and Drug Administration for the treatment of nosocomial pneumonia. Pump inhibitors are undergoing phase I trials in cystic fibrosis patients. Therefore, selecting appropriate antibiotics and optimising their use on the basis of pharmacodynamic concepts currently remains the best way of coping with pseudomonal infections.

Oral antibiotic therapy of serious systemic infections

Traditionally, antibiotics have been administered intravenously (IV) for serious systemic infections. As more potent oral antibiotics were introduced, and their pharmacokinetic aspects studied, orally administered antibiotics have been increasingly used for serious systemic infections. Antibiotics ideal for oral administration are those that have the appropriate spectrum, high degree of activity against the presumed or known pathogen, and have good bioavailability. Oral antibiotics with high bioavailability, that is > or = 90% absorbed, achieve serum/tissue concentrations comparable to IV administered antibiotics at the same dose. The popularity of "IV to PO switch therapy" is possible because of the availability of many potent oral antibiotics with high bioavailability. Initial IV therapy is appropriate in patients who are in shock/have impaired intestinal absorption, but after clinical defervescence, completion of therapy should be accomplished with oral antibiotics. As experience with "IV to PO switch therapy" has accumulated, confidence in oral antimicrobics for therapy of serious systemic infections has continued to increase. The trend in treating serious systemic infections entirely with oral antimicrobial therapy will continue, and is clearly the wave of the future.